The invention generally relates to nonlinear signal conversion, and, more particularly, to nonlinearly converting a digital signal into an analog signal while using pulse width modulation (PWM) to compensate for degradations in a display element of a display through a driver, such as a column driver of passive matrix (PM) displays.
Pulse width modulation has been employed to drive PM displays. In the PWM scheme, gray level information is encoded in the pulse width of the subpixel driving signal, while the pulse amplitude is used for overall brightness control (i.e., dimming) and non-uniformity/aging compensation. In such a scheme, gray level control is decoupled from dimming and compensation, leading to simple column driver architectures. A PWM scheme may control displays, including emissive and non-emissive displays, which may generally comprise multiple display elements. Emissive displays may include light emitting diode displays, liquid crystal displays, and organic light emitting device (OLED) displays, as examples.
In one approach, only linear mapping of gray level to subpixel intensity has been implemented. In high quality video displays, however, nonlinear mapping (e.g., gamma correction) may be required because of the nature of human eye""s nonlinear response. Namely, nonlinear compensation for the eye""s nonlinear response is desired.
Typically, pulse width modulation (PWM) entails using a signal to encode information in a pulse by switching the signal on or off as required. For example, in a particular PWM scheme, the amplitude of the signal at a particular instance determines the width of the pulse. A PWM scheme may control displays, including emissive and non-emissive displays, which may generally comprise multiple display elements. Emissive displays may include light emitting diode displays, liquid crystal displays, and organic light emitting apparatus (OLED) displays, as examples. In order to control such displays, the current, voltage or any other physical parameter that may be driving the display element may be manipulated. When appropriately driven, these display elements, such as pixels, normally develop light that can be perceived by viewers.
More specifically, in an emissive display example, to drive a display (e.g., a display matrix having a set of pixels), electrical current is typically passed through selected pixels by applying a voltage to the corresponding rows and columns from drivers coupled to each row and column in some display architectures. An external controller circuit typically provides the necessary input power and data signal. The data signal is generally supplied to the column lines and synchronized to the scanning of the row lines. When a particular row is selected, the column lines determine which pixels are lit. An output in the form of an image is thus displayed on the display by successively scanning through all the rows in a frame.
In a variety of displays, certain degradations of display characteristics may be caused by extended usage of display elements, manufacturing defects, and/or lack of calibration of display elements. For instance, when driving the pixels of a display over a period of time, the brightness of the pixels may deteriorate at different rates. Additionally, pixel aging may adversely affect the performance of the display (e.g., by reducing brightness of the complete display or some particular display elements). In such a display, it is not uncommon to find pixel non-uniformity attributable to display manufacturing defects. Thus, in certain cases initial non-uniformity, degradation over time, and non-uniform degradation generally needs to be compensated.
The human eye responds to ratios of intensities, not absolute values of intensities. The human eye perceives the difference between 0.1 and 0.11 as the same as the difference between 0.9 and 0.99, for example. Such behavior of the human eye is generally known as the gamma characteristic. Pertaining to displays, a gamma characteristic is defined as the rate at which gray levels transition from white to black. How the human eye perceives gray level transitions has always been a problem, since the human eye perceives different gray levels in a nonlinear fashion. Lack of the perfect gamma correction also affects color hues in a color display. Thus, a gamma correction may be needed to adjust for different xe2x80x9cwhitenessesxe2x80x9d of an image, which can create incorrect gray tones as perceived by the human eye. Similarly, since the eye is sensitive to relative contrasts, perceptible contrasts among the neighboring pixels will cause contouring effects and should be avoided. Hence, when discrete compensation is used, the fraction by which a compensation level increases from the immediately lower one should be smaller than a given value.
Unfortunately, conventional ways of driving displays using a typical PWM scheme may not be adequate. For example, conventional PWM schemes control display characteristics of particular display elements through width modulation or amplitude modulation to map video data to pixel brightness. Likewise, an appropriate compensation to overcome display distortions resulting from non-uniformity or aging of display elements may also be provided either by width modulation or amplitude modulation. However, it may be difficult to embed the gamma correction along with encoding display control and compensation-related information via simple, linear width modulation or amplitude modulation.
Thus, there is a need for better ways to controllably drive display elements in displays.